In order to achieve global mobility and a wide range access to offered mobile services for today's wireless radio systems and the users of these wireless radio systems, it is desirable to provide dual mode wireless radio systems. These systems are capable of operating with different wireless radio system standards, as for example, global system for mobile communication, GSM, enhanced data rates for GSM evolution, EDGE, universal mobile telecommunication standard, UMTS, or others. For generating, transmitting or receiving of radio frequency signals, nowadays digitally controlled oscillators, DCOs, are used. A DCO generates a radio frequency signal depending on a digital frequency word. Realized in a semiconductor device, a digitally controlled phase locked loop, PLL, with a DCO uses less space than a respective PLL with an analog voltage controlled oscillator, VCO.
In different standards, the wireless systems have different requirements on DCO performance. For a DCO specified for GSM the smallest frequency step size, that means the spacing between two tunable frequencies, should not be greater than 10 kHz, if no oversampling is used for controlling the DCO. The required frequency step size increases to about 100 kHz in case oversampling is used. In addition, the requirements for phase noise performance are stringent for GSM.
In contrast, for UMTS, the smallest possible frequency step size should be smaller than 200 kHz without oversampling, respectively, smaller than 500 kHz with oversampling. The requirements for phase noise performance for UMTS are moderate compared to GSM/EDGE.
To use a DCO in a direct phase modulator, it is practical to consider a required frequency step size in the DCO and a wanted modulation bandwidth. A fine resolution of the frequency step should be required to get an acceptable value for the error vector magnitude, EVM, of the output signal of the DCO. The modulation bandwidth is defined by the maximum frequency range required to achieve a modulation standard having the same number of available frequency steps. A smaller frequency step size results in a smaller achievable modulation bandwidth.
Wireless radio systems like GSM/EDGE have a limited modulation bandwidth of the output signal but high requirements for the error vector magnitude. UMTS systems have a broader modulation bandwidth but not the same restrictive requirements on the error vector magnitude.
DCOs known today are only adapted for a single standard. When integrating several wireless radio system standards into one device, the radio frequency blocks in the device should be shared as much as possible between different standards. This is especially desirable with shrinking structures of semiconductor technologies.